Fuel injectors often have piece-to-piece and time-to-time variability, due to imperfect manufacturing processes and/or injector aging, for example. Over time, injector performance may degrade (e.g., injector becomes clogged) which may further increase piece-to-piece injector variability. As a result, the actual amount of fuel injected to each cylinder of an engine may not be the desired amount and the difference between the actual and desired amounts may vary between injectors. Such discrepancies can lead to reduced fuel economy, increased tailpipe emissions, and an overall decrease in engine efficiency. Further, engines operating with a dual injector system, such as dual fuel or PFI/DI systems, may have even more fuel injectors (e.g., twice as many) resulting in a greater possibility for degradation of engine performance due to injector degradation. Diverse approaches may be used to estimate the variability in injector performance.
One example method is that used by Pursifull in U.S. Pat. No. 8,118,006 wherein injector variability in a dual fuel engine including a first and second fuel rail may be diagnosed by isolating one fuel injector at a time. For example, pumping of a second fuel into the second fuel rail is suspended while a first fuel is injected to all but a single cylinder of the engine, and, while pumping is suspended in the second fuel rail, the second fuel is injected into the single cylinder and corresponding pressure decrease in the second fuel rail is correlated to injector operation and possible degradation. Specifically, the measured pressure drop is compared with an expected decrease in pressure and associated with any of the following malfunctions: injector plugging, injector leakage and/or a complete failure of the injector.
The inventors herein have identified potential issues with the above approach. For example, during an injector calibration event, as an injector is tested, fuel rail pressure decreases. This decline in rail pressure may reduce injector backpressure causing an increase in injector closing delay and substantially affecting the accuracy of measured pressure drop. Further, if the calibration event occurs over a longer period of time, a larger overall drop in fuel rail pressure with a corresponding large increase in injector closing delay may affect the accuracy of pressure drop measurements substantially. Since measured pressure drops are correlated with an expected decrease to predict the presence (or absence) of injector degradation, inaccurate measurements can lead to incorrect conclusions about injector malfunction and subsequent fueling adjustments.
The inventors herein have recognized the above problem and have devised an approach to at least partially address it. In one example approach, a method is provided for measuring pressure drops with associated closing delays at each injection event and correcting the calculated pressure drops for an increase in injector closing delay. For example, in a multi-injector, dual-fuel system, a high pressure pump may be operated to temporarily raise pressure of a second fuel in a second rail coupled to an injector being tested. Once the pressure is at a predetermined level and after suspending pump operation, a single cylinder may be injected with a second fuel via a direct injector while other cylinders can be fueled with a first fuel via their respective port injectors. At each injection, a fuel rail pressure decrease may be measured along with an associated injector closing delay and then corrected for an increase in closing delay. For example, a percentage increase in closing delay at each rail pressure compared to that at base rail pressure may be calculated and the drop in rail pressure can then be corrected to accommodate for an increase in closing delay.
In this way, injector degradation, and consequently, injector variability, can be learned by measuring fuel rail pressure drops and adjusting these for a closing delay of an injector. In an engine with multiple injectors per cylinder, a single injector can be isolated for calibration by fueling all but one cylinder with a first fuel and injecting a second fuel into the single cylinder via the injector being calibrated. During the calibration, pressure drops, along with closing delays, associated with each injection event may be measured. By correcting for an increase in closing delay, a more accurate determination of pressure drop can be made, particularly when the overall pressure decline in the fuel rail is significant. As such, this corrected pressure drop will lead to a more precise adjustment in an amount of fuel delivered by the injector.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.